Sources Bugzilla – Bug 7065
Support building glibc with -fstack-protector or -fstack-protector-all
Last modified: 2012-04-21 12:55:37 UTC
Several distributions, including Gentoo, have patches which attempt to make
glibc build when compiled with -fstack-protector. None of them that I have
found actually work: at best, they make it crash instantly, because none of
them suppress generation of stack guards around functions called before the
guard is initialized (or even before ld-linux.so.2 has relocated itself).
For now I'd consider it more of a maintenance burden than it's worth to
isolate the parts of ld.so that are called before security_init(), so
I've just arranged to avoid using the stack-protector in ld.so, and
marked the few things not also in the dynamic loader which are used
during pre-guard-init static library initialization with
-fno-stack-protector. (An improvement might be to mark these with
-fno-stack-protector only when building the static library.)
I can't think of a way to isolate the functions in this set automatically, but
if you get it wrong it's easy to tell because you get an instant coredump, so
it's easy to prevent the set of -fno-stack-protectored functions from
bitrotting, manually maintained though it must be.)
You can still use -fstack-protector-all in all the rest of glibc, which
is an order of magnitude more code than ld.so and includes really hairy
stuff like malloc() and lots of functions that themselves call
string-manipulation functions, so I'd say this is a pretty good
Some of the module tests need adjustment: they don't link against
libc, so must specify -fno-stack-protector; the same is true of some of the
configury, taking care not to specify that unless GCC is actually capable of
accepting -fstack-protector. (We specify -lssp and leave the stack protector on
where possible in configure tests.)
Results of tests and patch against glibc 2.9 following shortly.
Never going to happen.
test environment: 188.8.131.52 kernel+headers, glibc 2.8, GCC 4.3.3 20081121
(prerelease), binutils 2.19.
config flags (my standard set for this machine):
/usr/packages/glibc/2.9/configure --prefix=/usr --enable-shared \
--enable-profile --disable-bounded --enable-bind-now \
--enable-add-ons=nptl,libidn --enable-kernel=2.6.25 \
--enable-check-abi=warn --enable-omitfp \
Below, PASS means 'baseline test failures only'. (I also compared
configure output to verify that -fstack-protector addition did not
change the results of any configure tests, and verified that the
appropriate -fstack-protector actually appeared in gcc commandlines at
the appropriate times.)
baseline: PASS, by definition
patch applied, no flags specified: PASS
-fstack-protector in CFLAGS: PASS
-fstack-protector-all in CFLAGS: one failure due to #7066, buffer overrun
--with-stack-protector=all: one failure due to #7066, buffer overrun
Test failures for baseline (unpatched):
testing long double (inline functions)
Failure: Test: expm1 (1) == M_El - 1.0
is: 1.71828182845904523532e+00 0xd.bf0a8b14576953500000p-3
should be: 1.71828182845904523543e+00 0xd.bf0a8b14576953600000p-3
difference: 1.08420217248550443401e-19 0x8.00000000000000000000p-66
ulp : 1.0000
max.ulp : 0.0000
Maximal error of `expm1'
is : 1 ulp
accepted: 0 ulp
Test suite completed:
3618 test cases plus 3005 tests for exception flags executed.
2 errors occurred.
--- ../scripts/data/localplt-i386-linux-gnu.data 2006-01-11
+++ - 2008-11-30 20:52:09.962033876 +0000
@@ -1,4 +1,5 @@
(This looks like something missing from localplt-i386-linux-gnu.data to
me, not a bug.)
(Holes in test coverage: not tested with a GCC too old to support
-fstack-protector. Static testing not performed: see #7064.)
Created attachment 3087 [details]
stack protector support for glibc
This is posted at the request of Carlos O'Donell. TBH I don't care if it
doesn't go upstream, although given that it's already found a buffer overrun in
glibc I'd find that surprising. I'm more interested in distros picking it up.
(And 'never going to happen' is a peculiar statement. It has 'happened'. The
A failure in elf/check-localplt.out as indicated by a new symbol in
scripts/data/localplt-i386-linux-gnu.data means that libc proper is invoking the
new symbol via the plt when in-fact it should be making a direct invocation to a
libc internal symbol. There are very few conditions under which this is allowed
(notably those cases where we allow libc functionality to be overridden).
I've looked through the code and it appears that the sunrpc code is the only
relevant code which uses __bzero. I don't think there's an internal hidden
version of the symbol. So these calls to __bzero probably shouldn't be there.
Instead, they should use memset. Or perhaps that patch which you're using uses
In order to verify, one can look at the symbol table:
objdump -DR libc.so > libc.dis
Search libc.dis for:
You should see a plt call stub, e.g.
16198: ff a3 0c 00 00 00 jmp *0xc(%ebx)
1619e: 68 00 00 00 00 push $0x0
161a3: e9 e0 ff ff ff jmp 16188 <h_errno+0x16168>
Now search for: "call 16198"
call 16198 <__bzero@plt>
This should bring you to the disassembly of the function which invoked __bzero
via the PLT. You can then go into the C source file and replace this with a
memset. Do this for all calls to the address for __bzero.
Here's a more thorough write-up of the same thing I just posted:
Invocation of __bzero() by the sunrpc code is acceptable since that code is in a
different library than libc.so so access via the PLT is expected.
Nice description, Ryan :)
A lot of the sunrpc code *does* land in libc (all the client code). Notably,
bindresvport(), clnt_create(), clnt_broadcast(), universal() (called from
registerrpc() via a callback from svc_register()), svctcp_create(),
svcudp_bufcreate(), and key_gendes(), _des_crypt() (obviously used for
DES-encrypted SunRPC), all explicitly call __bzero() and land in libc.
Almost certainly these would use memset() were the SunRPC code not ancient
Sun-derived gunge with a 1986 copyright date...
Keep it going, guys.
To Ulrich Drepper:
How do you mean: "Never going to happen."?!
I always keen on people ignoring security measures.
I've been using a compromise. Glibc's programs can be compiled
with -fstack-protector-all, or whatever other options you may want
(-D_FORTIFY_SOURCE=2, -fPIE, etc), but not the libraries. I use the configparms
file and set build-programs=no to build the libraries
without -fstack-protector, then remove build-programs=no and add 'CFLAGS
+= -fstack-protector-all' to configparms. No patches needed, test suites pass
(remove -fstack-protector during the test suite), no crashes.
Created attachment 6248 [details]
stack protector support against eglibc 2.13.
This is the most recent version of this patch, against eglibc 2.13 (because
that happens to be the version I'm using now, as I track Debian's glibc). It
has needed no significant revisions for years, though the recent csu changes in
upstream glibc may necessitate some small revisions.
(The ChangeLog is out of date: I haven't regenerated it since 2008.)
Your change contains two different changes:
* Supporting stack-protector
* A different implementation of chk_fail function
For addition to glibc, I would only look at the stack-protector support. I
suggest you continue discussing this on the libc-alpha list.